This invention relates to self-locking cable ties and more particularly to such a cable tie having a ladder strap and non-pivotal locking means for securely holding the strap.
Cable ties of molded thermoplastic material for conveniently forming elongate objects such as wires into a bundle have come into common use in the last twenty years. More recently, cable ties having stretched straps of reduced cross-sectional area have appeared. Since longitudinal stretching of the plastic strap to orient the molecules in the direction of the stretch causes a dramatic increase in tensile strength, such stretched straps are at least as strong as non-stretched straps which have a significantly greater cross-sectional area. It is also desirable to use a stretched strap in applications where a relatively long strap is needed because of the difficulties encountered in molding a long strap to its final dimensions. More particularly, increased heat and pressure are required to completely fill a long mold cavity and the increased heat may tend to cause degradation of the thermoplastic material while the increased pressure could cause the mold to pump open or flash. Examples of prior art molded ties are shown in U.S. Pat. Nos. 3,186,047 and 3,660,869.
Prior art self-locking cable ties typically include locking means pivotally carried by the head of the cable tie. Such locking means could either be integral with the head, such as a toothed-plastic pawl for engaging teeth on the strap, or non-integral with the head, for example, a metallic barb for biting into the strap. The integral locking means has an advantage in that it can be provided at the time of the molding and, accordingly, the number of operations required to manufacture the cable tie is reduced. Stretching the strap not only causes it to increase in tensile strength per unit cross-sectional area and also causes the thermoplastic material to significantly harden. As teeth formed on the strap at the time of molding tend to become distorted or "smear" during stretching, it is difficult for a prior art integral locking means to effectively engage such a stretched strap.
To overcome this disadvantage, a stretched strap of the ladder type has been proposed having longitudinally extending side rails joined by spaced transverse rungs. As opposed to the class of cable ties having non-integral metallic barbs which pierce or bite into the strap, the locking means used with a ladder strap extends between adjacent rungs. The locking means used with one prior art ladder strap is a long pivotal plastic finger pivotally joined to the head by a neck and engagable with a ledge upon attempted strap withdrawal. The neck must be sufficiently flexible to permit convenient threading of the strap into the head. Of course, the neck limits the strength of the finger and of the entire cable tie. An example of such a cable tie is shown in U.S. Pat. No. 3,766,608.
Straps of stretched thermoplastic material which are not self-locking have been proposed for bundling packages or a plurality of objects and for attaching tags and buttons to clothing. They typically require separate crimp connectors for application to an overlapping portion of the strap. Reference may be made to U.S. Pat. Nos. 3,444,597 and 3,447,207.